In recent years, in the manufacture of semiconductor elements and the like, advances in lithography techniques have lead to rapid progress in the field of miniaturization. Typically these miniaturization techniques involve shortening of the wavelength of the exposure light source. Until recently, ultraviolet radiation such as g-lines and i-lines have been used as the exposure light source, but recently, KrF excimer lasers (248 nm) have been introduced, and even ArF excimer lasers (193 nm) are now starting to be used.
Resists for use with light sources such as KrF excimer lasers and ArF excimer lasers require a high resolution capable of reproducing a pattern of minute dimensions, as well as good sensitivity relative to light sources with this type of short wavelength. One example of a type of known resist that satisfies these conditions is a chemically amplified positive type resist composition comprising a base resin that displays increased alkali solubility under the action of acid, and an acid generator that generates acid on exposure (see patent reference 1)
In the reaction mechanism of a chemically amplified resist, exposure causes the acid generator within the resist to generate acid, and this acid causes a change in the solubility of the resin. For example, if a dissolution inhibiting group that dissociates in the presence of acid is introduced into the resin, then this dissolution inhibiting group will dissociate only within the exposed sections of the resist, causing a significant increase in the solubility of the resist in the developing liquid within these exposed sections. Typically the dissociation reaction of the dissolution inhibiting group is accelerated by conducting a post exposure baking (PEB) treatment. Furthermore, the PEB treatment also promotes the diffusion of acid within the resist, meaning a much higher sensitivity can be achieved than with conventional non-chemically amplified resists.
In KrF excimer laser lithography, polyhydroxystyrenes or derivatives thereof in which the hydroxyl groups are protected with an acid dissociable, dissolution inhibiting group, which display high transparency relative to a KrF excimer laser (248 nm), have been used as the base resin component of chemically amplified resists. However, these resins display unsatisfactory transparency near 193 nm, and are essentially unusable for ArF excimer laser lithography. Accordingly, current base resins for ArF resists utilize a (meth)acrylic polymer comprising, as an acid dissociable, dissolution inhibiting group, an aliphatic polycyclic hydrocarbon group with a polycyclic skeleton such as an adamantane structure, and with a tertiary carbon atom within this skeleton.
In recent years the degree of miniaturization has progressed rapidly, and nowadays, resolutions capable of generating line and space patterns of less than 100 nm and isolated patterns of no more than 70 nm are being sought. As a result, in addition to the research and development being conducted on resist materials to enable ultra-miniaturization, research is also being conducted on pattern formation methods to develop techniques capable of overcoming the resolution limits of resist materials.
One example of such a technique, which has resulted in a number of different proposals, is a method in which a resist pattern is first formed using photolithography, and subsequent heat treatment is then used to further reduce the size of the resist pattern.
For example, the patent reference 2 discloses an omission pattern formation method in which an omission pattern is first formed in a pattern formation resist applied to the surface of a substrate, a mixing generation resist that mixes with the pattern formation resist is then applied across the entire surface of the substrate, baking is performed so that a mixed layer is formed on the side walls and the surface of the pattern formation resist, and the unmixed sections of the mixing generation resist are then removed, enabling the pattern size to be reduced by the dimensions of the mixed layer.
Furthermore, the patent reference 3 discloses a pattern formation method in which a resist pattern comprising an acid generator is formed on a substrate, the entire surface of the substrate is coated with a resin that becomes insoluble in the presence of acid, a heat treatment is then conducted, causing acid to diffuse from the resist into the resin, forming a resist layer of uniform thickness at the interface between the resin and the resist pattern, and developing is then used to remove those sections of the resin into which the acid has not diffused, thereby enabling the pattern size to be reduced by the dimension of the aforementioned uniform thickness.
Furthermore, recently, thermal flow processes in which the resist pattern is fluidized through heat treatment or the like, thereby enabling a reduction in the pattern size, have also been proposed. In a thermal flow method, a resist pattern is first formed using photolithography, and by subsequently heating the pattern to a temperature exceeding the glass transition temperature (Tg) of the resin component within the resist layer, thereby softening the resist, the size of the resist pattern is reduced.
For example, the patent reference 4 discloses a method of forming a fine pattern in which a resist pattern is formed on a substrate, heat treatment is conducted, and the cross sectional shape of the resist pattern is changed from a rectangular shape to a semicircle, thereby increasing the length of the base and forming a finer pattern.
Furthermore, the patent reference 5 discloses a method of forming a fine pattern in which following formation of a resist pattern, heating is conducted to approximately the softening temperature of the resist pattern, and fluidization of the resist causes a narrowing of the pattern size.
(Patent Reference 1)
Japanese Unexamined Patent Application, First Publication No. 2002-162745
(Patent Reference 2)
Japanese Unexamined Patent Application, First Publication No. Hei 5-166717
(Patent Reference 3)
Japanese Unexamined Patent Application, First Publication No. Hei 5-241348
(Patent Reference 4)
Japanese Unexamined Patent Application, First Publication No. Hei 1-307228
(Patent Reference 5)
Japanese Unexamined Patent Application, First Publication No. Hei 4-364021
As described above, in those cases where heat treatment is performed on the resist following resist pattern formation (and following developing), the pattern size of the resist pattern, namely, the size of the sections in which the resist is not formed (such as the hole diameter within a hole pattern or the space width within a line and space (L&S) pattern) reduces relative to the size prior to the heat treatment. In such cases, if a conventional ArF resist is used, the degree to which the pattern size reduces (the degree of narrowing) differs for differing patterns even on the same substrate, resulting in a problem that resist patterns of different pattern sizes can be formed on a single substrate (namely, variation can develop in the narrowed resist pattern size).